Frequency-dependent attenuation of a transmitting medium can be corrected by a filter with an adjustable frequency characteristic. In adjusting such a filter there is required knowledge of the attenuation properties of the channel in question and a new adjustment is required for alterations in the channel, e.g. alterations due to re-disposition of the cable etc. Should there be an alteration in the channel, there is an obvious risk that the filter adjustment is forgotten or that an adjustment is incorrect. To avoid these difficulties, filters are used with a variable frequency characteristic, which can be controlled in cable transmission by a DC current applied to the line, the strength of the current being dependent on the length of the line. Since the channel signal attenuation is not unambigously related to the strength of the DC current, compensation will be incomplete, resulting in that a transmitted signal can be wrongly detected.